1000 Hydraulics Chapter - Ministry of Transportation

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BC MoT SUPPLEMENT TO TAC GEOMETRIC DESIGN GUIDE MoT Section 1040 TAC Section Not Applicable 1040.05 CULVERT DESIGN EXAMPLE Problem The design flow (Q 200 ) for the creek has been estimated as 7.0 m 3 /s. The creek slope at the highway is -2.5%. Determine the culvert size and flow characteristics. Solution Step 1 - Preliminary Culvert Dimension and Hydraulics The minimum pipe grade for inlet control and initial dimensions for SPCSP culverts can be determined using Figure 1040.B. A worked example (see Figure 1040.D) provides the following: 3 Q200 = 7.0 m / s D = 2200 mm 2 Ac = 2.4 m 3 Q capacity = 7. 8 m / s Sc = 0022 . m / m n = 0.031 Step 2 - Check Headwater Depth The final culvert slope is -2.5%. The slope is steeper than the critical slope ensuring that the culvert will operate under inlet control. If the culvert were to be placed on a milder slope (say -1.0%), outlet control may govern and a backwater analysis would be required to determine the headwater depth. Referring to Figure 1040.F (Example 2), if we assume a 2.28 m diameter SPCSP with the entrance mitered to conform to the slope (k e =0.7) and a design flow of 7.0 m 3 /s: HW = D 082 . HW = (0.82)(2.28 m) = 1.87 m The inlet configurations satisfy the inlet control design criteria which requires that the headwater depth to diameter ratio (HW/D) not exceed 1.0 at the design flow. Step 3 - Determine Full Flow Characteristics Using Manning’s Equation and the Continuity Equation, the full flow characteristics of the culvert can be determined. See Table below D A R=D/4 R 0.67 n S S 0.5 /n v full =R 0.67 S 0.5 /n Q full =vA (m) (m 2 ) (m) (m/m) (m/s) (m 3 /s) 2.28 4.1 0.57 0.69 0.031 0.025 5.1 3.5 14.4 Step 4 - Determine Partial Flow Characteristics Since the culvert is operating under inlet control, the flow within the barrel will be partially full. Partial flow characteristics for the culvert were determined using a hydraulic element chart for a circular pipe. Q 7.0m 3 / s = Qfull 14.4 m 3 = 049 . / s d D = 05 . d = (0.5)(2.28 m) = 1.1 m v vfull = 10 . v = (1.0)(3.5 m / s) = 3.5 m / s The average flow velocity in the culvert (v) should be used for outlet riprap design, while the average depth of flow (d) may be used for outlet control calculations. Page 1040-12 June, 2007
BC MoT SUPPLEMENT TO TAC GEOMETRIC DESIGN GUIDE MoT Section 1050 TAC Section Not Applicable 1050 PAVEMENT DRAINAGE AND STORM SEWERS 1050.01 RETURN PERIOD For design return periods, refer to Section 1010.02. 1050.02 PAVEMENT RUNOFF The runoff for highway pavements is computed by the Rational Formula Method using a runoff coefficient (C) equal to 0.95 and a minimum time of concentration equal to 5 minutes. 1050.03 PAVEMENT GRADES The desirable minimum sustained grade for curbed pavements is -0.5%, with -0.3% allowed for a curb and gutter section as an absolute minimum. If a level grade on a low-speed curbed road is unavoidable, false grading of the gutter may have to be provided to produce an absolute minimum slope of -0.3% to the inlets. Roadway design should try to limit the number of lanes which drain in one direction. 1050.04 PONDING WIDTHS Gutters should generally be designed such that the maximum ponding width at the catchbasin or spillway is equal to 65% of the paved shoulder width with a minimum of 1.2 m. For low grade roadways, the ponding width may have to be increased to maximize the inlet spacing. However, encroachment of the gutter flow onto the traveled portion is discouraged due to the possibility of hydroplaning, soaking of pedestrians etc. Ponding widths should be measured from the face of the curb. For information on gutter flow, refer to: ♦ RTAC Drainage Manual Volume 2 (1987), p. 5.22. 1050.05 MEDIANS AND CURBS Median drainage is generally designed for a maximum depth of 0.3 m. Drainage curbs and outlets for paved surfaces on erodible slopes will be required if any of the following criteria are met: • Fill height exceeds 3 m high. • Longitudinal grade is greater than 4%. • Superelevation is over 6%. • Any superelevated pavement is wider than 15 m. Asphalt curbs are generally used for rural projects. Concrete curbs are used for urban projects and other areas where there is considerable development. 1050.06 GRATES/SPILLWAYS Grate Inlets The current practice is to use Bicycle Safe grates on any roadway which cyclists are permitted to travel. Freeway grates shall be used on all other roadways. Wide pavements tend to require depressed grate inlets. Similarly, urban areas use depressed grate inlets. Due to vane configurations, twin Bicycle Safe and Freeway grate inlets are recommended in areas where gutter flow velocities exceed 1.5 m/s and 2.0 m/s respectively. Table 1050.A presents grate catchment widths which are recommended for use with the Spreadsheet or Calculator Method grate inlet spacing calculations: Table 1050.A - Grate Catchment Widths Inlet Type w (m) Undepressed Bicycle Safe 0.305 Depressed Bicycle Safe 0.625 Undepressed Freeway 0.375 Depressed Freeway 0.625 For drawings of catchbasin grates, refer to: • MoT Standard Specifications for Highway Construction, Drawing No. 10-SP219. Spillways In rural areas with potential pavement debris problems (e.g. debris from deciduous or coniferous trees, heavy sanding operations, etc.) spillways may be preferred over catchbasins. Spillway channels lined with riprap are generally recommended. Paved spillway channels are not recommended unless they have very short lengths and adequate soils supporting the sides of the channel. It should also be noted that spillways are more susceptible to damage from snow clearing operations. June, 2007 Page 1050-1
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1000 Hydraulics Chapter - Ministry of Transportation

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